KFC 150 Strange behavior altitude hold / preselect.

[redbaron1982]

I have an strange behavior with my KFC150 altitude hold mode / altitude preselect function.

While testing in the ground everything "seems" (the quotes just because I cannot test everything) to be OK. If I press ALT in KFC150 it will iluminate, if I then set a VS in the preselect window it will go out. If I "ARM" the alt preselect it will stay ARM and (by adjusting the alt setting) I can simulate a change of altitude and the mode will go from ARM to CAPT and then to altitude hold whenever the selected altitude is reached.

So again, in ground everything makes sense.

After takeoff, the ALT hold mode in the KFC150 will not activate, the altitude preselect will not ARM (and of course no CAPT either). Only VS mode works OK in flight.

Otherwise, the HDG, NAV, APR (except for GS) modes work OK.

Does anyone have any idea why on the ground it might seem to at least make sense and while flying it won´t even engage?

I'm trying to pin down what might be going on before taking in to a shop, as some of you might know I have had a first annual inspection that set me back 1yr and 40AMU, so I'm being as cheap as possible now!

[81X]

Are you running the preflight test before launching?   From what I recall, failure to do so before flight CSM lend to some funny behavior happening. 

[redbaron1982]

26 minutes ago, 81X said:

Are you running the preflight test before launching?   From what I recall, failure to do so before flight CSM lend to some funny behavior happening. 

Yes, so after I turned on the avionics I do the AP test, then I play with the alt preselect and everything looks ok, without turning anything off I take off and then no more alt hold mode or alt arm/Capt.

[LANCECASPER]

14 minutes ago, redbaron1982 said:

Yes, so after I turned on the avionics I do the AP test, then I play with the alt preselect and everything looks ok, without turning anything off I take off and then no more alt hold mode or alt arm/Capt.

Since the airplane is new to you, have you flown the KFC150 before? Each of these autopilots has it's own button-ology. If you don't have a lot of time with the KFC150 or that wasn't part of your transition training it might be good to get with an instructor that knows that autopilot.

You probably already have the KFC150 pilot's guide, but just in case (starting with pg. 16 especially, follow every step without skipping anything):   https://www.bendixking.com/content/dam/bendixking/en/documents/downloads/006-08377-0001_1-kap-150-ki-525a-pilot-s-guide.pdf

I would get the basics of the KFC150 down first before you get too far into the KAS-297B (pre-select), starting at pg. 89

[carusoam]

Read up… probably something you are missing…. 90s technology and all…

Then find Jake…

Best regards,

-a-

[redbaron1982]

Thanks guys. I'll re read the operating guide. But I do remember flying a KAP150 sometimes in flight school and don't remember having this issue.

Also the two instructors that help me with the transition training couldn't figure this out either.

Hopefully it's just me having to learn the right sequence of buttons to press so I'll read it again.

[redbaron1982]

Hey guys, I've read (again) the KFC 150 operative manual, and I cannot find what I'm doing wrong on my side. Although I might be missing something, the operation seems to be straightforward. For instance, on yesterday's flight, this was the sequence I did (regarding the AP):

• After turning on the avionics, I performed an AP self-test (by pressing the test button). Which passed.
• After takeoff, above 1000ft AGL I engaged the NAV mode (by pressing the FD button, then the NAV button, and finally the AP ENGAGE button) mode to follow the course defined by the 430W, and the AP started to follow it.
• After that, I tried to engage ALT hold mode by pressing the ALT button, and it did not engage.

If anyone knows a special sequence key I need to press to get this engage or tell me what I'm doing wrong, I'd appreciate your input.

[carusoam]

Great details RB!

Lets invite @Jake@BevanAviation to stop by…  (KFC150 alt hold challenge)

Best regards,

-a-

[LANCECASPER]

I had a KFC150 in a Mooney Bravo that did the same thing (would follow nav or hdg but not hold alt) years ago under warranty. it happened a few times and every time they would send the KC192 back to Bendix King and they would provide a loaner which would work and then when I got the original one back from service it failed again on the next flight. Finally at my request they let me keep the loaner - problem solved.

 

@Jake@BevanAviation

[carusoam]

I wonder if there is a capacitor slowly degrading that is responsible for alt hold…

The KFC series might be more modern than the old capacitor prone devices…

KAP/KFC150s are early digital compared to the KFC200…

Best regards,

-a-

[jkgyr52]

FWIW - Have you tried using the Approved Flight Manual Supplements (AFMS) in the POH for operating the KFC 150 and 297B ?

On page 5 of the Pilots Guide there is a statement that reads: "This Pilot's guide provides a general description of various operational characteristics of the KFC150, KAP150 and KAP100 Flight Control Systems. However, operation of a system should not be attempted without reviewing the specific information in the FAA approved Aircraft Flight Manual Supplement for your particular aircraft type."

Perhaps this will help you figure out if operation is the problem of the computer is wonky.

I have had two K models with the KFC 150/297B system and have had too many issues with porpoising, altitude hold and capture. The avionics technician has repaired bad connections on the board, capacitors and alt hold module. My current airplane just had the computer OH so if I have any more issues with it I am considering replacing it with another autopilot.

 

[Jake@BevanAviation]

The logic in the computer for the mode buttons is pretty simple, 5VDC pull up resistors tied to the button switch that pulls the line low when the button is depressed.  After that it runs into 8bit shift register IC and then the microprocessor.  If I had to guess maybe a issue with the shift register or possibly the microprocessor.  I would try and isolate out the KAS297B and see if the issue stops.  

Also, KC192 computers manufacture between 1991 and 1998 have capacitor leakage problems.  There should be a red or black stamp on the side/back of the unit that has a 4 digit code for example 1393 would be the 13th week of 1993.  I would try to see if your unit falls in the manufacture range.   When the capacitors leak they can cause some really weird issues with the flight computer. 

[redbaron1982]

I hope it's a leaking capacitor, it should be fairly easy to fix if that's the case. I will take a look at the date the unit was manufactured to see if it falls inside these dates.

[Pinecone]

I have KAP-150 with KAS-297B.

I occasionally get some porpoising. The way I found to stop is easiest is to hit the CWS for a bit.  

Mine will hold an altitude if I hit the ALT button at that altitude.  But it will only capture if I set a VS number for the climb of descent.

[LANCECASPER]

1 hour ago, redbaron1982 said:

I hope it's a leaking capacitor, it should be fairly easy to fix if that's the case. I will take a look at the date the unit was manufactured to see if it falls inside these dates.

i believe it ruins the board in some cases when it leaks, that's why that date range is not desirable.